The present invention relates to an internally cooled high-energy cable, and more particularly to a water cooled high-voltage high-energy cable having a closed internal cooling channel.
There are already known high-energy cables including a plurality of radially superimposed layers of various properties which surround one another and an internal cooling channel. For transmitting energies in the order of 2000 MVA, it is already known to provide a cable which is formed with an internal cooling channel bounded by the electric conductor itself. Experience with this type of cable has shown that for the electric conductor of aluminum the diameter of the cooling channel should be greater than 60 millimeters, particularly equal to or exceeding 70 millimeters.
It is also already known to use water as the cooling medium for internally cooling the high-energy cable. However, many problems are encountered when water is used as the cooling medium. So, for instance, when the cooling channel is bounded by the electric conductor itself as mentioned above, that is when the innermost layer of the cable is of a material having high electric conductivity, such as aluminum, there exists the danger that the electric conductor will be attacked by the cooling medium, that is cooling water, and will corrode over a period of time until the cable is rendered useless.
Further difficulties are encountered when the electric conductor is constituted by a plurality of layers, some of which are circumferentially complete tubular electric conductors and some of which are constituted by layers or segments of elongated electrically conductive elements which surround or are surrounded by the tubular electric conductors. Such a multi-layer construction of the electric conductor is often necessary, particularly where, as in the present case, the thickness of the electric conductor in the radial direction of the cable is substantial, in order to permit bending of the electric conductor during the manufacture, transportation and laying of the cable. The difficulties arising from such a construction are particularly pronounced when the elongated electrically conductive elements are of the same material as the tubular electric conductor. Namely, only a certain length of the cable can be transported to the point of use, and the cable is usually assembled from a plurality of such lengths in situ by welding the end portions of such lengths to one another. During the welding, the inner tubular elements are welded to one another first, with the elongated electrically conductive elements removed from the region of welding, and then another welding operation is performed for connecting the elongated electrically conductive elements of the adjacent lengths of the cable to one another to form the superimposed layer of the electric conductor which surrounds the inner tubular electric conductor. It will be appreciated that, during the second welding operation, the previously manufactured welded connection of the two adjacent tubular electric conductors will be reheated to the welding temperature, that is to a temperature which at least plasticizes the material of the welded connection of the tubular electric conductors. As a result of this reheating of the welded connection, the quality thereof in most instances suffers, so that it is impossible or at least very difficult to assure a faultless water-tight connection between the two inner tubular electric conductors.
Another aspect to be taken into consideration when manufacturing such cables is that the materials of the various layers of the electric conductor must be so selected that the danger of electric interaction of these materials is kept to the minimum. More particularly, the electric potentials of these materials must be as close to each other as possible so as to prevent or minimize damage to the electric conductor resulting from these materials acting as an electric cell. The same consideration is also valid for the characteristic properties of the other members of the cable and its connecting arrangements which come into contact with the cooling medium.
In the above-mentioned electric cables, there also exists the danger that, due to the relatively low resistance to wear on the electric conductor bounding the cooling channel through which the cooling water or similar cooling fluid flows at relatively high speeds, the erosion of the internal surface of the electric conductor will be relatively high which will render the electric cable of the prior art useless within a relatively short period of time, especially after the cooling fluid starts leaking through the electric conductor.